1. Field of the Invention
The present invention relates to a technology for applying a lubricant to an image carrier in an image forming apparatus.
2. Description of the Related Art
Recently, a demand for forming high-quality images in an image forming apparatus, such as a copier, a facsimile, or a printer disclosed in, for example, Japanese Patent Application Laid-open No. 2007-272091, is growing. To fulfill this demand, toner that contains more fine and spherical particles is being developed. Hereinafter, such toner will be referred as fine and spherical toner. The reason behind this is that a dot-reproducibility increases as the size of the toner particles decreases, and a development performance and a transferability increase as the sphericity of the toner particles increases. However, the fine and spherical toner can hardly be produced by the conventional mixing and crushing method. Therefore, polymerized toner produced by a suspension polymerization method, an emulsion polymerization method, or a dispersion polymerization method is becoming popular as the fine and spherical toner.
However, use of the fine and spherical toner leads to the following problems when cleaning an image carrier such as a photosensitive drum after forming images.
It is difficult to scrap the fine and spherical toner with a cleaning blade that is typically employed for scrapping toner from the image carrier in the image forming apparatus. Specifically, when the cleaning blade scrapes off toner from the surface of the image carrier, the cleaning blade is sometimes deformed due to a friction between the image carrier. In case the cleaning blade is deformed, toner is caused to roll in a space between the image carrier and the cleaning blade, so that some toner remains behind on the image carrier. If the amount of toner that remains behind on the image carrier increases, a cleaning failure occurs, resulting in degradation of images such as occurrence of background fog in the images.
Furthermore, the toner that has rolled out of the cleaning blade may be remained on the surface of the image carrier leading to filming. The filming is a phenomenon that mold release agent or fluidizer contained in the toner adheres to the surface of the image carrier as a film. The filming leads to degradation of images such as occurrence of white points in one-colored area of the images.
To improve the performance of the cleaning blade, one approach is to reduce a coefficient of friction of the surface of the image carrier. Such a technique has been disclosed in, for example, Japanese Patent Application Laid-open No. 2002-207397, Japanese Patent Application Laid-open No. 2005-062709, Japanese Patent Application Laid-open No. 2004-325621, and Japanese Patent Application Laid-open No. 2006-251751. Specifically, the coefficient of friction of the surface of the image carrier is reduced by forming a thin film of a lubricant made of metallic salts of fatty acids or the like on the surface of the image carrier. When the coefficient of friction is small, adherence of the toner to the image carrier decreases. As a result, the cleaning performance of the cleaning blade can be improved and the filming can be prevented.
Japanese Patent Application Laid-open No. 2002-207397, Japanese Patent Application Laid-open No. 2005-062709, Japanese Patent Application Laid-open No. 2004-325621, and Japanese Patent Application Laid-open No. 2006-251751, teach to use a brush to apply a lubricant to the image carrier. However, if images with large image portions are formed continuously, a large amount of toner may be attached to the brush. In this situation, when the brush receives the lubricant, the lubricant may stick to the toner adhering to the brush. As a result, the performance of applying the lubricant to the surface of the image carrier drops. The above problem is more likely to occur in the configuration disclosed in Japanese Patent Application Laid-open No. 2002-207397 and Japanese Patent Application Laid-open No. 2005-062709, in which a lubricant applying device is arranged at a position upstream of a position where an image carrier is cleaned in a moving direction of the image carrier.
On the other hand, a configuration in which a lubricant applying device is arranged at a position downstream of a position where an image carrier is cleaned in a moving direction of the image carrier is disclosed in Japanese Patent Application Laid-open No. 2004-325621 and Japanese Patent Application Laid-open No. 2006-251751. With this configuration, the above problem is less likely to occur. Furthermore, because the lubricant is applied to the image carrier that has been cleaned, application of the lubricant to the image carrier does not depend on what images have been formed on the image carrier.
However, in the configuration as disclosed in Japanese Patent Application Laid-open No. 2004-325621, when a brush unit only applies the lubricant and does not flatten the applied lubricant on the image carrier, functions of the applied lubricant cannot be obtained sufficiently. On the other hand, Japanese Patent Application Laid-open No. 2006-251751 discloses a configuration in which a flattening unit that flattens the lubricant applied to the image carrier is arranged. However, arrangement of the flattening unit leads to an increase in costs and size of an apparatus.
To flatten the lubricant applied to the image carrier in a compact apparatus, it is preferable to flatten the lubricant on the image carrier while the configuration as disclosed in Japanese Patent Application Laid-open No. 2002-207397 and Japanese Patent Application Laid-open No. 2005-062709 is assured, in which the lubricant applying device is arranged at a position upstream of a position where the image carrier is cleaned in the moving direction of the image carrier and the image carrier is cleaned by the cleaning blade. However, this configuration leads to degradation of the performance of applying the lubricant to the image carrier as described above.
In the configuration disclosed in Japanese Patent Application Laid-open No. 2005-062709, a unit that removes toner from the brush unit is provided. However, because this unit removes the toner at a position between a position where the brush unit receives the lubricant and a position where the lubricant is applied to the image carrier in a rotation direction of the brush unit, the lubricant may be removed from the brush unit while the toner on the brush unit is removed. Therefore, the performance of applying the lubricant may degrade. In the configuration disclosed in Japanese Patent Application Laid-open No. 2007-272091 and Japanese Patent Application Laid-open No. 2004-325621, bias is applied to the brush unit. However, a unit for applying the bias needs to be added, which leads to increase in costs.